The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to magnetic resonance angiography (MRA) and methods for increasing the acquisition speed of MRA studies.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as “spin-warp”. The spin-warp technique is a well-known technique that employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then an echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed. In a typical 3DFT pulse sequence spatial information is encoded along two orthogonal axes and the phase encodings (ΔGy and ΔGz) are both stepped through values to sample Fourier space, or “k-space” in a prescribed manner.
There are many other k-space sampling patterns used by MRI systems These include “radial”, or “projection reconstruction” (PR) acquisitions in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space. The pulse sequences for a radial scan are characterized by the lack of a phase encoding gradient and the presence of a readout gradient that changes direction from one pulse sequence view to the next. There are also many k-space sampling methods that are closely related to the radial scan and that sample along a curved k-space sampling trajectory rather than the straight line radial trajectory.
An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then perform a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered backprojection.
MR angiography (MRA) is the application of magnetic resonance imaging methods to the depiction of the human vasculature. The non-invasiveness of MRA makes it a valuable screening tool for cardiovascular diseases. To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. Excellent diagnostic images may be acquired using contrast-enhanced MRA (CEMRA) if the data acquisition is properly timed with the bolus passage. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CEMRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds.
The short separation time between arterial and venous enhancement dictates the use of acquisition sequences of either low spatial resolution or very short repetition times (TR). Short TR acquisition sequences severely limit the signal-to-noise ratio (SNR) of the acquired images relative to those exams in which longer TRs are possible. The rapid acquisitions required by first pass CEMRA methods thus impose an upper limit on either spatial or temporal resolution.
Efforts have been made to acquire CEMRA images in shorter scan times using undersampled projection reconstruction scanning methods. As described in U.S. Pat. No. 6,487,435, it was discovered that image artifacts due to k-space undersampling are unsubstantial when radial acquisitions are used. This is particularly true of CEMRA image frames in which a pre-contrast mask image is subtracted from each acquired image frame.
Many different strategies have been developed to shorten the scan time. In fact, ever since the initial development of MRI in the 1980s there has been considerable interest in methods that are “accelerated” to yield a reduction in acquisition time. One early example is partial Fourier imaging, which is a method that exploits the mathematical symmetry of the MRI data to allow a reduction in the number of phase encoding views that are acquired to sample k-space. One such method is to partially acquire k-space and then calculate the missing data. Such “partial” Fourier data acquisition usually uses Hermitian conjugate symmetry to replace the missing k-space data. Hermitian conjugate symmetry only works if the image is real. Numerous phase errors are present in MRI data that make the image complex. These phase errors result from phenomena such as B0 inhomogeneity, gradient eddy currents, group delays in the gradient amplifiers and receive electronics, and the spatial variation of surface coil receive B1 fields. To enable Hermitian conjugate replacement to work with a complex image, the replacement of the missing k-space data is accompanied by a phase correction that removes the phase errors from this data. One partial Fourier reconstruction algorithm, called “Homodyne reconstruction”, uses two filters to accomplish the Hermitian conjugate replacement and the phase correction, respectively. A Homodyne high-pass filter doubles the amplitude of the acquired k-space data which is conjugate to the missing k-space data prior to the Fourier transform. After the Fourier transform, the imaginary part of the image is discarded to complete the replacement step. The phase correction step is accomplished by a Homodyne low pass filter. This filter creates an image from a small portion of k-space data acquired symmetrically around the center of k-space. The phase of this image is subtracted from the phase of the Homodyne high pass filtered image prior to discarding the imaginary part of the image.
Another example of acceleration that is applicable to time-resolved imaging is referred to as “view sharing.” View sharing is a technique in which some MR measurements are shared from one reconstructed image to the next, allowing for an image frame rate which is accelerated above normal.
Yet another general method for acceleration exploits the multiple signals obtained if multiple receiver coils are used. Such methods of parallel acquisition or “parallel imaging” allow up to N-fold acceleration for data from N coils. Parallel imaging techniques use spatial information from arrays of RF receiver coils to substitute for the encoding that would otherwise have to be obtained in a sequential fashion using RF pulses and field gradients (such as phase and frequency encoding). Each of the spatially independent receiver coils of the array carries certain spatial information and has a different sensitivity profile. This information is utilized in order to achieve a complete location encoding of the received MR signals by a combination of the simultaneously acquired data received from the separate coils. Specifically, parallel imaging techniques undersample k-space by reducing the number of acquired phase-encoded k-space sampling lines while keeping the maximal extent covered in k-space fixed. The combination of the separate MR signals produced by the separate receiver coils enables a reduction of the acquisition time required for an image (in comparison to conventional k-space data acquisition) by a factor that in the most favorable case equals the number of the receiver coils. Thus the use of multiple receiver coils acts to multiply imaging speed, without increasing gradient switching rates or RF power.
Parallel imaging techniques fall into one of two categories. They can fill in the omitted k-space lines prior to Fourier transformation, by constructing a weighted combination of neighboring lines acquired by the different RF detector coils. Or, they can first Fourier transform the undersampled k-space data set to produce an aliased image from each coil, and then unfold the aliased signals by a linear transformation of the superimposed pixel values.
Two such parallel imaging techniques that have been developed and applied to in vivo imaging are SENSE (SENSitivity Encoding) and SMASH (simultaneous acquisition of spatial harmonics). Both techniques include the parallel use of a plurality of separate receiving coils, with each coil having a different sensitivity profile. The combination of the separate NMR signals produced by these coils enables a reduction of the acquisition time required for an image (in comparison with conventional Fourier image reconstruction) by a factor which in the most favorable case equals the number of the receiving coils used.
A number of these “acceleration” methods can be integrated to synergistically provide an acceleration factor higher than what is attainable by any method individually. For example, partial Fourier imaging can be used with SENSE acceleration for a net acceleration factor higher than the number N of coil elements used. Also, view sharing can be used with more frequent sampling of the k-space center to provide an increased frame rate.
As view sharing is carried to an extreme in which case the k-space center is updated far more frequently than peripheral k-space, the averaging of data that occurs over the necessarily long temporal footprint ultimately causes temporal blurring or smearing of time-varying phenomena. Also, as the degree of undersampling of the k-space is made more extensive, the resultant images can have artifactual patterns because of severe undersampling.
It would therefore be desirable to have a system and method for accelerating imaging acquisitions but that does not suffer from the drawbacks of the above-described methods or combinations of drawbacks.